@33Hp that is because some skills need a specific allowed target to get activated.
Same problem you have if there is a group of people killing a creature and standing too close to them, so that you can't target the creature, that means also, you probably can't hit them with ranged weapons.
The only valid Trigger for that skills are the Attack Symbol at mouse pointer.
If you don't have it, you can't perform.
That makes me thinking about that this funktion is surely nice in a PVP later, because Tanks can guard healer this way a bit better, especially because we also have a collision control.
But it is hard in PVE for targetting Mobs.
I think that will be a mayor task for Devs to make both working.
As for Structures like Trees and Houses i would still see, that they break your Skills, since you are not in line of sight anymore. But for sure, since we just have one view to the world, trees will be under mouse even you will have a line of sight.
Probably it would help here, if the interaction with objects is always on right mouse, while Attacking is always left.
Then a combination of all possible actions at the position of your mouse, even they overlap.
In PVP of course it can be hard to balance, but a line of sight check would help much to prevent abusing it, like casing a physical spell to a target, if another is in front. But it is already like this, that the first target in line of sight gets this spell.
Non physical are another thing.
